This invention relates to a process for treating paper mill waste water and, more particularly, to a method of coagulating the suspended material in paper mill waste water so that it may be easily removed therefrom and the clarified water reused in the paper making process.
For years paper mills of all types, both primary and secondary mills, have faced serious problems in obtaining an adequate supply of fresh water and in disposal of the waste water effluents. Historically paper mills have taken huge amounts of water from local supplies, used the water in various segments of the paper making process, combined the waste water from these operations, and then discharged the effluents into nearby lakes, streams and rivers. For that reason, the Environmental Protection Agency has recently issued effluent guidelines and standards for the pulp, paper and paperboard point source category (39 FR 18742, May 29, 1974).
While means are known for treatment of paper mill waste water to bring the effluents in line with the EPA specifications, these involve primary, secondary and tertiary treatments which are time consuming, costly and for the most part impractical. As stated in Gavelin, Fourdrinier Papermaking, Lockwood Trade Journal Co., New York, New York, 1963, at page 27:
"The load of fiber, filler and dissolved organic material in the effluents remains a surprisingly acute problem in most paper mills. It is surprising because with modern technology it would seem a trivial matter to clean water, at least to the extent of removing suspended material. The explanation to the mystery is the need for economy." The economic consideration in the water flow of a paper mills also involves the need for procuring additional fresh water to the extent that waste water cannot be economically treated and reused.
In primary and secondary paper mills the need for useable water is considerable. The fresh water needs and effluent problems vary somewhat depending on the type of paper making operation, but all paper mills have need for an economical process for treating and recycling waste water.
Primary mills may be defined as those operations that more or less continuously make paper from pulp produced at the mill itself. In the unbleached kraft process, for example, the water used to wash the pulp after the cooking operation is returned to the recovery loop for the pulping process as black liquor. This black liquor is concentrated and burned to recover the dissolved salts.
The washed pulp which then goes to the paper making process still contains considerable dissolved lignin together with other organic color bodies. Fresh water used in dilution, refining, and other processes picks up the dissolved lignin, sizing, fines, etc. in passing through the system. The resultant waste water is a dark brown murky fluid, smelling of mercaptans and other odoriferous chemicals from the pulping process.
Secondary mills are defined as those operations that make paper, boxboard, roofing paper and the like, as the result of procuring the raw materials elsewhere, i.e., virgin pulp, reclaimed pulp, waste paper, etc.
For example, in a boxboard mill the product consists of several layers of paper laminated together. The outer layers are usually white and the inner layers are usually darker. The pulp requirements for the various layers differ.
The top liner sheet is usually white and may be made from bleached virgin pulp or produced by repulping white waste cuttings. The water requirements for this layer in the bulking, refining and dilution steps are that the water be substantially clear of suspended solids and emulsions and be substantially colorless.
The under liner and back liner are usually of less quality and the water requirements are less stringent than the top liner. In practice the effluent from the top liner section is used as the supply for these laminae. Additionally, the pulp requirements for these laminea are less and are typically obtained by repulping and deinking newspapers, magazines, etc.
The filler sections require an even less quality water and pulp. The water supply usually comes from the under liner and back liner sections. The pulp is often derived from repulping and cleaning common mixed paper waste.
The water used in the various pulpings, refinings, deinkings, cleanings, and dilutions, becomes contaminated with a number of substances. Among these are: starches, caseins, ink particles, carbon particles, clay, resins, fines (small cellulose fibers), and emulsifying agents. The result is a waste water that has a slow settling portion, suspended solids, dissolved solids, and is generally murky and foul smelling. As such it is unsuitable to be reused in the paper making process.
Attempts have been made to clarify the settleable fraction and recirculate some of the water obtained from the clarification process back to the liner sections. This has resulted in lowering the strength of the board. Furthermore, it is insufficiently clean and clear to use in the top, back and under liner sections. In addition, the waste water cannot be readily filtered since the various components (gels, etc.) quickly "blind" a filter. Therefore, a relatively coarse screen is used resulting in the passage of a substantial amount of fines which in turn reduces the overall strength of the board. In cases where this method has been used, increasing the caliper of the board has been necessary to compensate for the reduced lamina strength.
Chemical treatments have also been used in conjunction with centrifuging, filtering, sedimentation, etc., in an attempt to clarify paper mill waste water. These include the use of alum, lime, and organic polyelectrolyte flocculating agents. One such process in use at the Sterling Operation of Brown Company adds activated silica, alum and sulfuric acid to the waste water prior to its entering the clarifier. The chemical additives are said to provide floc formations and enhance the coagulation and settleability of the solids to the bottom of the clarifier. The clarified water is then released into a river while the sludge is pumped to a filter house where polymers are added to assist in the removal of water from the sludge.
The patent literature reveals that a number of such systems have been proposed for clarification of industrial waste effluents of various types. For example, Chappell in U.S. Pat. No. 3,812,032, discloses a system for treating domestic, municipal and industrial liquid wastes to reduce the BOD of the effluent. The Chappell process involves using an acid solution to adjust the pH to about 0 to 4 and an alkaline formula to adjust the pH to about 9-14. However, the only disclosed embodiment for the use of the acid and alkaline formulas is to separate the waste water into two portions. The first portion is treated with the acid formula and the second portion is treated with the alkaline formula, and then the two are mixed. Despite Chappell's asserted success with this order of addition of chemcial formulas, it has been found that in treating paper mill waste water such an arrangement does not result in good floc formation and clarification sufficient for recycling the water.
Other patents also suggest using acid and alkaline formulas but with different orders of additions. For example, in U. S. Pat. No. 3,314, 880 to Rubin there is disclosed a method of purifying waste water containing proteinaceous substances by adding an acid to give the liquid a pH value of 4 - 4.5 followed by addition of a basic substance. This addition of chemicals is done during a flotation process during which other precipitants are also added.
Likewise, in U.S. Pat. No. 3,354,028 to Illingsworth there is disclosed a method of clarifying the effluent from a waste paper deinking plant by chemically reversing the pH on addition, in either order, of alum and an alkali or alkaline earth metal hypochlorite. In the acidification phase, the pH is lowered to 4.0 - 6.5 and in the basic phase the pH is raised to 6.5 - 7.5.
British Pat. No. 1,032,725 also discloses a system using an acid formula and an alkaline formula in the treatment of industrial sewage. Here, ferrous sulfate in the form of a 5-25% solution in dilute hydrochloric acid (pH 1 - 2.5) is added, supplemented by a sufficient amount of sodium hydroxide to obtain a pH value in the liquid to be clarified in the range of 7.5 - 9.0.
Similarly, Gaughan in U.S. Pat. No. 3,575,853 discloses a process of removing multivalent metal ions from waste water by adding acid, passing the acidified aqueous medium through a bed of metal particles, and then raising the pH with an alkaline material. The acid step lowers the pH to from 1 to 5 and the addition of alkaline material raises the pH from 7 to 10.
It has been found that the problem with these procedures is that unless the pH is lowered to below 4, there be thorough mixing, and then the pH raised to above 10, the process is not effective to clarify paper mill waste water sufficiently to permit its reuse in the paper making process.
Accordingly, there remains a need for an effective, low-cost process for treating paper mill waste water to clarify it adequately for reuse in the paper making process.